I'm thinking GTA: San Andreas-like Autoduel. I love SA partly because of the cars and the fact they had a million and one things to do in this huge world.

It took me forever to unlock the whole map because I would always get sidetracked by just goofing off in the world or some other little sidemission/challenge/races/goofing off by picking people up in a taxi. Slap some automobile combat on that sucker, a few more rpg-lite skills, maybe some C&C (not a must, but wouldn't hurt), tournaments of death, some transport missions, upgradable cars that don't fracken disappear when you do a mission and I'm in high heaven.